Having accumulated promising young players like Brandon Ingram, left, in recent drafts, the key to the Lakers’ rebuild now turns to this summer’s free agency window, when they find out whether the new-look front office can lure elite stars such as Paul George, right. (Photo by Harry How/Getty Images)

It’s not that they’re rebuilding, which even dynasties have to do. The problem is that it’s going so slowly, heading toward their fifth season in a row out of the playoffs. Their longest absence before had been two seasons … once … 41 years ago.

Now that everyone around here has learned to be r-e-a-l-l-y patient, the question is what the Lakers will get out of this.

Of course, we won’t know the answer for years but here’s how it’s going in Rebuilding Land.

IN A LEAGUE OF THEIR OWN

Boston — No, really.

Just because you’re good now doesn’t mean you’re not capable of building for a better future if everything goes just right as it has for GM Danny Ainge. The Celtics are leading the East despite losing Gordon Hayward with the best likely yet to come as elite prospects arrive annually from promising Jaylen Brown, 21, to star-in-waiting Jayson Tatum, 19, with one more lottery pick from the treasure trove that Ainge got from Brooklyn’s Bill King for 37-year-old Kevin Garnett and 36-year-old Paul Pierce in one of the biggest heists in NBA history.

Making it all the more painful to their former archrivals out here, Boston’s last pick from the Nets deal might be the one conveyed from the Lakers to the Suns to the 76ers in the 2012 trade for Steve Nash.

One way or another, the Lakers won’t have their first-round pick in June. If it falls between Nos. 2 and 5 – an 80 percent probability as things stand – it will transfer to the 76ers and then on to the Celtics for trading their 2017 No. 1 overall pick to Philadelphia, which took Markelle Fultz, knowing they would get Tatum at No. 3.

So that roar you hear every time the Lakers lose isn’t the surf, it’s Celtic fans.

NEXT BEST THINGS

Minnesota — No. 4 in the West with Karl-Anthony Towns, 22, Andrew Wiggins, 22, and enough other prospects to trade Zach Lavine and Kris Dunn for Jimmy Butler.

Milwaukee — Coach Jason Kidd prefers veterans but Giannis Antetokuonmpo, 23, the game’s best player under 25, is a one-man rebuilding program.

Denver — Fighting for a playoff berth is remarkable for a low-budget operation whose team was 33-49 two seasons ago, now featuring the game’s best passing center, 22-year-old Nikola Jokic.

NOT QUITE YET

Philadelphia — The 76ers slid to 5-10 last month but given good health – a question with Joel Embiid – no one has a young core as spectacular as Embiid, 23; Ben Simmons 21; Fultz, 19, and Dario Saric 23.

Miami — Young as they are, they’re a tenacious Pat Riley-Erik Spoelstra Heat team, but their star, Hassan Whiteside, is 28 and a knucklehead who’s always in trade rumors. … With South Beach and Riley’s cachet, he’ll try to pull off one of his free agent coups as he did with LeBron James.

Indiana — For an unhappy Paul George, the Pacers nabbed happening Victor Oladipo, 25, and Domantas Sabonis, 21, from OKC to go with shot-blocking stretch five Myles Turner, 21. They’re now doing better without PG than they did last season with him.

Utah — If you’re having a lost season as they are post-Hayward, this is the kind to have as No. 13 pick Donovan Mitchell breaks out with fabulous-for-a-rookie numbers (18 points per game, shooting 44 percent, 36 percent from 3-point range).

THINGS HAVE BEEN WORSE

Lakers — This was supposed to be the season they got to 35 wins before pursuing LeBron, Paul George, et al.

As awful as their record is, their nucleus is significantly advanced over a year ago when their top prospects were immature D’Angelo Russell, limited shooter Julius Randle and the frail, not-NBA-ready Ingram.

Happily – or hopefully – for them, the key isn’t accumulating more players in the draft but free agency when they find out what magic Magic Johnson is capable of.

Knicks — Another painful sight for the Lakers: 7-foot-2 Kristaps Porzingis, 22, whom they could have drafted instead of Russell, emerging as a go-to guy, which the Lakers lack. It’s why they lose all those close games. … The Knicks (18-20) are doing OK for a team coming off a 32-50 season but are subject to the whims of clueless owner James Dolan. Many – including LeBron – asked why the new administration that succeeded Phil Jackson took Frank Ntilikina instead of Dennis Smith Jr.

Suns — Sharpshooting Devin Booker, 21, who posted a 70-point game last season, might be the best young player this side of Antetokuonmpo, averaging a fancy 32-5-5 last month. T.J. Warren, 24, is averaging 19.7 points. … A lot depends on big, athletic, 3-point-shooting, shot-blocking intermittently-active power forward Marquese Chriss, 20.

Dallas — If their only comer is Dennis Smith Jr., 20, owner Mark Cuban and GM Donnie Nelson are sharp guys capable of building from here.

NOWHERE MEN

Chicago — Dunn (22) and Lavine (23) will be a nice backcourt, but this is another weak organization under what-me-worry owner Jerry Reinsdorf, even if he lucked into six titles with Michael Jordan.

Orlando — The Magic are paying for former GM Rob Hennigan’s mistakes after hiring him out of the OKC front office at 30, such as Oladipo and Tobias Harris trades with no one left from those deals. Aaron Gordon, 22, is their lone comer.

Memphis — The Grizzlies are in free fall but have yet to start moving out veterans such as Marc Gasol and Mike Conley. This franchise is a good bet to be somewhere else – like Seattle – by the time it’s back in the playoffs.

Atlanta — Oft-sold franchise is now down to one good young player to build around, Dennis Schroder, 24.

A LEAGUE OF THEIR OWN

Brooklyn — New GM Sean Marks landed inexpensive prospects Russell, 21, and Jahlil Okafor, 22, but that hardly makes up for dealing all their lottery picks since 2013.

Sacramento — New owner Vivek Ranadive loved telling people how he learned the game late in life coaching his daughter’s grade school team. Now to explain how he has rebuilt the Kings for four seasons, traded DeMarcus Cousins and doesn’t have one happening prospect to show for it.

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